1. Field of the Invention
The present invention relates to a structure in which a surface-normal optical (or photonic) device or material is mounted on an optical fiber or waveguide, in which the surface-normal optical device or material has a function of controlling the intensity, phase, polarization of light, or a function of receiving, emitting, or modulating light. In particular, the present invention relates to a technique for inserting a thin and surface-normal active optical device into a trench which is formed perpendicularly to a substrate on which an optical fiber or waveguide is mounted.
2. Description of the Related Art
Optical communication using optical fibers has been rapidly spreading because it can transmit large amounts of data at high speed.
Optical waveguides are used in order to perform separating, coupling, switching, wavelength-division multiplexing, or wavelength-division demultiplexing of light. Optical waveguides are made of glass or polymeric material and are thus basically passive devices. However, the refractive index of an optical waveguide can be partially changed by providing a local heater or the like so as to obtain a thermo-optical effect. Accordingly, the phase and polarization of light can be controlled, thereby realizing an optical switch, variable optical attenuator, variable optical filter, or the like.
However, when a heater is provided on a substrate for waveguides, the distance between the heater and the core of the waveguide is large. Therefore, if a plurality of heaters having required high power are provided on a substrate, the temperature of the entire substrate is increased.
In addition, when a device or material having a function of emitting or processing light is provided at an optical waveguide, conventionally, the device or material is mounted on the optical waveguide. More specifically, when a semiconductor chip or the like having such a function is mounted on an optical waveguide, a relevant portion of the waveguide is removed, and the semiconductor device (i.e., chip) is mounted on that portion in parallel to the surface of the waveguide. Therefore, the distance between the divided waveguides is large, and thus transmission loss is large. In addition, it is very difficult to adjust a core (through which light passes) of the semiconductor chip to the core of the waveguide.
When a surface-normal optical device such as a semiconductor laser or an optical detector is mounted on an optical waveguide, the device is put on the same surface of the optical waveguide and the direction of light is changed by 45xc2x0 by using a mirror. This structure is suitable for forming electrodes and being integrated. However, the distance between the device and the waveguide is large and the light is diffused; thus, a condenser such as a micro lens is necessary.
In addition, a technique for inserting a passive optical device such as a filter or a wavelength plate (typically, a half- or quarter-wave plate) into a trench formed in the waveguide is known. However, when an active surface-normal optical device is inserted into such a trench, necessary electrodes cannot be formed and obtained.
In most conventional optical devices used for optical communication, light output from an optical fiber is collimated so as to make the light pass through a surface-normal optical device, which can be selected from various kinds of surface-normal optical devices (i.e., optically-functional devices). This light is collimated again using a collimating lens so as to allow input into an optical fiber for outputting the light. However, many problems occur in this case, for example, the surface-normal optical device is relatively large and is expensive.
FIGS. 38A to 38D show typical optical devices using collimating fibers, where each collimating fiber has a collimator.
In the figures, reference numeral 19-1 indicates an optical fiber having an input collimator, reference numeral 19-2 indicates an optical fiber having an output collimator, reference numeral 19-3 indicates a rotatable half-wave plate, reference numeral 19-4 indicates a rotatable quarter-wave plate, and reference numeral 19-5 indicates a rotatable or movable ND filter. Reference numeral 19-6 indicates a homogeneous liquid crystal device, a TN liquid crystal device, a liquid crystal variable-wavelength filter in which a liquid crystal is inserted in a Fabry-Perot interferometer, or a piezo-controlled Fabry-Perot interferometer-type variable-wavelength filter. Reference numeral 19-7 indicates a first polarizer, reference numeral 19-8 indicates a second polarizer, and reference numeral 19-9 indicates a Faraday rotator.
In typical polarization control devices, a quarter-wave plate and a half-wave plate are inserted between the collimating fibers, and the polarization state of incident light can be changed by rotating such wavelength plates without any limitation (refer to FIG. 38A). The wavelength plates are manually rotated in most laboratories. However, in practical systems, motor-controlled rotation is employed.
In variable optical attenuators, mechanical attenuators are known, in which a planar ND filter is rotated or moved with respect to collimated incident light (refer to FIG. 38B). The planar ND filter can be manually adjusted or can be controlled using a motor.
The following devices are also known: (i) phase modulators in which a liquid crystal having a homogeneous alignment is inserted between collimating lenses, (ii) polarization switching devices for switching the polarization direction between 0xc2x0 and 90xc2x0, in which a TN liquid crystal is inserted between collimating lenses, (iii) liquid crystal variable-wavelength filters in which a liquid crystal is inserted in a Fabry-Perot interferometer-type filter, and (iv) Fabry-Perot interferometer-type variable-wavelength filters in which a filter gap is adjusted using a piezo element (refer to FIG. 38C).
Additionally, known optical isolators have a structure in which a Faraday rotator is inserted between polarizers whose polarization directions differ by 45xc2x0 from each other, where collimated light is transmitted between the polarizers (refer to FIG. 38D). In order to provide polarization-insensitive characteristics in this structure, a polarization separating element must further be employed.
In addition, optical fiber amplifiers are known, in which an excited optical beam is input into an optical fiber via a free-space optical beam system.
In the above-explained conventional devices, it is generally difficult to perform coupling and adjustment of the optical beam. In addition, such devices generally provide a single channel system, and the device is large. Therefore, it is difficult to reduce the costs of relevant optical elements.
When a trench having a width of 10 to 100 xcexcm is formed in a substrate on which an optical waveguide or an optical fiber is provided and a functional device as explained above is vertically inserted into the trench, the optical device (including the functional device) which can be realized in a free-space optical beam system can also be realized as a waveguide-type device. In such a small width of 10 to 100 xcexcm, radiation loss of light due to the presence of the trench is smaller than the power loss in the free-space optical beam system. In particular, if the width of the trench is equal to or less than 40 xcexcm, the radiation loss is very small, approximately 0.2 dB.
The inventors have realized a variable optical attenuator by filling a trench, which is formed in a substrate on which an optical waveguide or fiber is fixed, with a liquid crystal material. In optical waveguides, a wavelength plate made of polyimide may be inserted so as to cancel the polarization dependence, or a dielectric mirror formed on a polyimide material may be inserted so as to perform wavelength-division multiplexing of light. That is, a liquid or an elastic material can be relatively easily inserted into a trench as explained above.
However, when a solid surface-normal optical device which is made of glass, semiconductor, electro-optic crystal, ceramics, or the like and has a thickness of 10 to 50 xcexcm is inserted into a corresponding narrow trench, the device tends to be damaged, and thus it is very difficult to handle the device. If a micro-positioning stage is used for insertion of the device, the adjustment is very difficult. Even when the micro-positioning stage is erroneously moved by 1 xcexcm, the surface-normal optical device may be damaged.
In addition, if the trench is formed using an etching method such as RIE (reactive ion etching), the depth of the trench is shallow, typically, 50 to 100 xcexcm. In this case, even if a thin device can be inserted, the position of the inserted device is not stable and tends to fall.
Furthermore, even if a thin optical device having a thickness of 10 to 50 xcexcm can be inserted into a trench, it is difficult to form electrodes on the inserted device.
In consideration of the above circumstances, an object of the present invention is to provide a technique for electrically mounting a surface-normal optical device or material on a waveguide-type optical device while the characteristics of the mounted optical device are effectively used.
Another object of the present invention is to provide a technique for easily mounting a thin surface-normal optical device having a thickness of 10 to 100 xcexcm on an optical device in which an optical fiber or waveguide is provided on a substrate.
The above and other objects, and distinctive features of the present invention will be shown below with reference to the drawings.
Therefore, the present invention provides a waveguide-type optical device comprising:
a substrate on which optical waveguides or optical fibers are provided and a trench for dividing optical paths of the optical waveguides or the optical fibers is formed;
a pair of electrodes which is assigned to each optical waveguide or optical fiber and is formed from the surface of the substrate at both sides of the trench to wall surfaces of the trench; and
a material or device which is filled or inserted into the trench, and which has one of an electro-optic effect, a thermo-optic effect, a light emitting function, a light receiving function, and a light modulating function.
The electrodes may be extended by attaching a flexible substrate or by wire bonding, and a voltage may be applied to the material or device via the extended electrodes.
The followings are typical examples of the material or device which is filled or inserted into the trench:
(i) one of a nematic liquid crystal having an electro-optic effect, a cholesteric-nematic phase transition type liquid crystal, a polymer network liquid crystal a polymer-dispersed liquid crystal, a polymer-stabilized liquid crystal, a dynamic scattering liquid crystal, and a ferroelectric liquid crystal,
(ii) a polymeric material having a thermo-optic effect, and
(iii) one of a surface-normal optical modulator, a surface light emitting device, and a planar (i.e., surface-normal) detector which has one of a light emitting function, a light receiving function, and a light modulating function.
If the material or device which is filled or inserted into the trench is the polymer-dispersed liquid crystal, preferably, the polymer-dispersed liquid crystal is one of a normal polymer-dispersed liquid crystal in which each particle has a diameter of 0.5 xcexcm or more, and a nanosize droplet liquid crystal in which each particle has a diameter of 150 nm or less.
The present invention also provides a manufacturing method of a waveguide-type optical device, comprising the steps of:
forming a trench on a substrate on which optical waveguides or optical fibers are provided, in a manner such that the trench divides optical paths of the optical waveguides or the optical fibers;
forming a pair of electrodes, which is assigned to each optical waveguide or optical fiber, from the surface of the substrate at both sides of the trench to wall surfaces of the trench; and
filling or inserting a material or device into the trench, which has one of an electro-optic effect, a thermo-optic effect, a light emitting function, a light receiving function, and a light modulating function.
Typically, the electrodes are formed by sputtering or vapor deposition.
The step of forming a pair of electrodes may include:
(i) the steps of inserting a polymer material into the trench and selectively removing a portion of the polymer material; and performing patterning of said pair of electrodes, which is separately assigned to each optical waveguide or optical fiber, on the wall surfaces of the trench by etching, or
(ii) the step of patterning the electrodes On the wall surfaces of the trench by directly using a laser beam.
When a liquid crystal is filled into the trench, the filling step may include:
coating each wall surface of the trench with an alignment layer for the liquid crystal;
performing rubbing of the alignment layer by inserting a tape and pulling the tape in a single direction;
filling a polymer-stabilized liquid crystal into the trench;
performing alignment of the liquid crystal by irradiation of ultraviolet light while a magnetic field is applied to the liquid crystal.
Also when a liquid crystal is filled into the trench, the filling step may include the steps of coating each wall surface of the trench with a photo-alignment layer for the liquid crystal; and performing alignment of the liquid crystal by irradiating the photo-alignment layer with first and second polarized ultraviolet light beams.
Also when a liquid crystal is filled into the trench, the filling step may include the steps of coating each wall surface of the trench with an alignment layer for the liquid crystal; and performing alignment of the liquid crystal by irradiating the alignment layer with an ion beam.
According to the present invention, a surface-normal optical device or material can be inserted into a trench of a waveguide-type device, and the electrodes of the surface-normal optical device or material can be extended to the surface of the waveguides via the pairs of electrodes formed from the surface of the substrate to the wall surfaces of the trench. Therefore, the surface-normal optical device or material and the waveguide-type device can be electrically mounted while the characteristics of each device are effectively used.
In addition, each pair of the electrodes (transparent electrodes or metal electrodes) can be formed on the wall surfaces of the trench by sputtering or vapor deposition. Therefore, the electrodes can be formed while no short circuiting is caused on the bottom face of the trench.
The material or device inserted into the trench can be flexibly selected so as to realize a specific device for controlling the intensity, phase, and polarization of light or a device for controlling emitting or receiving light.
When a liquid crystal is filled into the trench, the alignment of the liquid crystal can be controlled.
The present invention also provides a waveguide-type optical device comprising:
a substrate on which optical waveguides or optical fibers are provided and a trench for dividing optical paths of the optical waveguides or the optical fibers is formed;
a thin and surface-normal active optical device driven by an applied voltage, which is substantially vertically inserted into the trench and is fixed in the trench; and
a support member attached to the thin and surface-normal active optical device.
The following explanations relate to this type of waveguide-type optical device.
Preferably, for a given thickness w of the thin and surface-normal active optical device, width W of the trench satisfies the condition xe2x80x9cw less than W less than 300 xcexcmxe2x80x9d.
Electrodes may be formed on the support member, which function as electrodes of the thin and surface-normal active optical device.
Preferably, the support member is one of a rectangular block, an L-shaped block, and a cylindrical block, and the block is made of one of glass, ceramics, and plastics; and height h and width I of the block, and length s of a protruding portion of the thin and surface-normal active optical device, which protrudes from the block, have a relationship of xe2x80x9cI/h greater than s/Ixe2x80x9d by which the thin and surface-normal active optical device does not fall when the device supported by the support member is put on the surface of the substrate in an inclined position.
As a typical example, the thin and surface-normal active optical device has electrodes;
the support member is a rectangular block, and L-shaped electrodes are formed on the block in a manner such that the L-shaped electrodes lie on two adjacent faces of the block, where the faces include the top face of the block; and
the electrodes of the thin and surface-normal active optical device are respectively connected to the electrodes of the block attached to the device, thereby extending the electrodes of the device to the top face of the block.
Typically, the thin and surface-normal active optical device is one of:
a PbS optical detector formed on a glass plate or an a-Si optical detector;
an optical detector obtained by thinning a semiconductor device;
a semiconductor optical modulator;
a polarizer obtained by dispersing metal particles in glass, where the particles are aligned in the long particle axis;
a wavelength plate made of an optical crystal;
a dielectric multi-layered filter deposited on a glass plate;
an ND filter;
a variable-wavelength filter made by placing an electro-optic crystal or electro-optic ceramics between dielectric multi-layered mirrors; and
a polarization modulator having an electro-optic crystal or electro-optic ceramics.
If the thin and surface-normal active optical device is a liquid crystal device, the support member may be a pair of blocks between which the liquid crystal device is placed, wherein the liquid crystal device may comprise:
thin glass plates which are respectively attached to faces of the blocks, where said faces of the blocks face each other via the liquid crystal device and a patterned electrode is formed on each glass plate;
an alignment layer formed on each thin glass plate, where the alignment layer is subjected to an alignment process such as rubbing; and
a liquid crystal filled into a space between the alignment layers of the thin glass plates.
It is possible that:
the patterned electrode includes 8 electrodes having a radial and symmetric form with respect to a center portion surrounded by the 8 electrodes;
said center portion has a window having a diameter of 20 to 50 xcexcm;
voltage applied to each of the 8 electrodes is controlled so as to apply an electric field, which has any desired power and is in any desired direction, to the center portion surrounded by the 8 electrodes; and
incident light having any polarization direction is converted into light having any desired polarization direction.
It is also possible that:
the thin and surface-normal active optical device is a thin optical modulator which comprises:
a thin PLZT plate having four trenches dug from upper, lower, right, and left sides of the plate;
four electrodes formed from the above four sides of the PLZT plate to the inside of each trench;
a conductive adhesive with which each trench is filled; and
a thin glass plate attached to the PLZT plate, which has four electrodes to which the four electrodes of the PLZT plate are respectively connected, and
wherein the thin glass plate is attached and fixed to the support member in a manner such that light passes through a center portion between the four electrodes of the PLZT plate, and the electrodes of the thin glass plate function as external electrodes of the optical modulator; and
voltage applied to each of the four electrodes is controlled so as to apply an electric field having any desired power and in any desired direction, thereby continuously and completely controlling the polarization direction of incident light into light having a linear polarization.
In this case, preferably, the optical waveguides or optical fibers which are provided on the substrate are expanded core fibers, so as to reduce the radiation loss of light.
Regarding this type of waveguide-type optical devices, the present invention provides a manufacturing method of a waveguide-type optical device, comprising the steps of:
forming a trench on a substrate on which optical waveguides or optical fibers are provided, in a manner such that the trench divides optical paths of the optical waveguides or the optical fibers;
attaching a support member to a thin and surface-normal active optical device which is driven by an applied voltage, in a manner such that a portion of the active optical device protrudes from the support member; and
substantially vertically inserting the protruding portion of the thin and surface-normal active optical device which is supported by the supported member into the trench and fixing the device in the trench.
In a preferable example, a positioning mark is provided on the thin and surface-normal active optical device before the device is inserted into the trench to make a portion of the thin and surface-normal active optical device, through which light passes coincide with a corresponding core of each optical waveguide or optical fiber, where the position of the positioning mark is away from the position of the portion through which light passes, by the distance from the surface of the substrate to the position of the core; and
the support member is attached to the thin and surface-normal active optical device in a manner such that the positioning mark coincides with the bottom face of the support member.
In another preferable example, the support member is one of a rectangular block, an L-shaped block, and a cylindrical block, and the block is made of one of glass, ceramics, and plastics;
height h and width I of the block, and length s of a protruding portion of the thin and surface-normal active optical device, which protrudes from the block, have a relationship of xe2x80x9cI/h greater than s/Ixe2x80x9d; and
the step of inserting the protruding portion of the thin and surface-normal active optical device includes the steps of:
putting the device supported by the support member on the surface of the substrate in an inclined position, so as to prevent the device from falling onto the substrate;
sliding the device on the surface of the substrate towards the trench; and
making the device fall into the trench and fixing the inserted device,
In the step of sliding the device on the surface of the substrate, both the support member and an end of the thin and surface-normal active optical device may contact the surface of the substrate.
Typically, in the step of making the device fall into the trench, when the thin and surface-normal active optical device reaches the position of the trench, an end of the device contacts a wall surface of the trench and the thin and surface-normal optical device bends and falls into the trench.
In a preferable example, the thin and surface-normal active optical device has electrodes; and the support member is a rectangular block, and the method further comprising the steps of:
forming L-shaped electrodes on the block in a manner such that the L-shaped electrodes lie on two adjacent faces of the block, where the faces include the top face of the block; and
respectively connecting the electrodes of the thin and surface-normal active optical device to the electrodes of the block attached to the device, thereby extending the electrodes of the device to the top face of the block.
According to the latter type of the waveguide-type optical device and the manufacturing method therefor according to the present invention, the following effects can be obtained:
(1) A polarization control device for converting light having any polarization into light having a linear polarization can be formed on a substrate on which optical waveguides or optical fibers are provided.
(2) High-speed phase modulation can be performed, thereby realizing a high-speed optical waveguide switch.
(3) A variable-wavelength filter for selecting a specific wavelength can be formed on a substrate on which optical waveguides or optical fibers are provided.
(4) An optical attenuator can be formed on optical fibers or optical waveguides.
(5) It is possible to monitor the intensity of passing light.
(6) Various kinds of optical devices, which are conventionally realized in a free-space optical beam system, can be realized on a substrate on which optical waveguides or optical fibers are provided, so that the size of the waveguide-type optical devices can be reduced.
As explained above, the inventors of the present invention invented that in the step of making a thin surface-normal optical device fall into the trench, when the surface-normal optical device is slid on the substrate by using tweezers or the like and the device reaches the trench, an end of the device contacts a wall surface of the trench and the thin and surface-normal optical device bends and falls into the trench, thereby easily inserting the surface-normal optical device into the trench. In this method, unnecessary force is not imposed on the device, thereby preventing the device from being damaged.
Also as explained above, preferably, a positioning mark is provided on the thin and surface-normal active optical device before the device is inserted into the trench, where the position of the positioning mark is away from the position of the portion through which light passes, by the distance from the surface of the substrate to the position of the core. Accordingly, when the device is inserted into the trench, it is possible to make a portion of the device, through which light passes, coincide with a corresponding core of each optical waveguide or optical fiber.
If a micro-positioning stage or the like is used for insertion of a surface-normal optical device into a narrow trench, unnecessary force tends to be imposed on the device, thereby damaging the device.
In addition, the thin and surface-normal active optical device is attached to the support member such as a block; therefore, in addition to reinforcement effect, the entire surface-normal optical device can be subjected to photo processing or the like. Furthermore, if the surface-normal optical device is a liquid crystal device, the device can be coated with an alignment layer or be subjected to rubbing or the like. If a thin and planar chip is attached to a thin glass plate having electrodes and the glass plate is further adhered to a glass block, complicated electric wiring of the planar chip can be connected and extended to the electrodes of the block.
Japanese Unexamined Patent Application, First Publication No. Hei 9-297229 xe2x80x9cProduction of Filter Type Waveguidexe2x80x9d discloses a structure in which a filter to which a block is attached is inserted into a trench which passes across a waveguide device. However, this filter is a passive device such as a wavelength plate and thus is not a surface-normal active optical device having electrodes. In addition, the block attached to the filter is provided for correcting a camber of the filter or making the position of the filter coincide with the position of the trench, rather than for functioning as a support member. In the present invention, owing to the support member, when the surface-normal optical device supported by the supported member reaches the trench and contacts a wall surface of the trench, the device can bend and be easily inserted into a trench without using a positioning marker (which is used in Hei 9-297229 for making the position of the filter coincide with the position of the trench).
S. Kawakami et al., xe2x80x9cVertical Photonics: A New Approach to Integrate Photonic Devices into Optical Fibersxe2x80x9d, the Proceeding of IEICE, C-I, Vol. J77-C-I, No. 5. pp. 334-339, 1994, discloses a structure in which a liquid crystal device is inserted so as to pass through an optical fiber array provided on a substrate. However, in this structure, the thickness of the device is 600 xcexcm or more; thus, no concept of using a block is disclosed and the object, structure, and function of the above structure differ from those of the present invention.